Three separate columns were seen — one near the main separatist stronghold of Donetsk and two outside the town of Snizhne, 80 kilometers (50 miles) further east. The vehicles were mainly transportation trucks, some of them carrying small- and large-caliber artillery systems, and at least one armored personnel carrier. Several of the trucks were seen to be carrying troops.

Ukrainian officials said this week that they believe rebel forces have received substantial consignments of weaponry and manpower from Russia. Moscow denies such claims.

It was not immediately possible to establish the provenance of the vehicles seen Saturday. Separatists have always insisted they are armed with equipment captured from Ukrainian forces, but the sheer scale and quality of their armaments have strained the credibility of that claim.

Ukrainian National Security and Defense Council spokesman Volodymyr Polevoy said rebel reinforcements have also been observed moving toward front-line locations around 150 kilometers (95 miles) northeast of Donetsk, in the Luhansk region.

Polevoy said rebel authorities are boosting their ranks by forcibly mobilizing residents in a number of occupied towns.

Despite a cease-fire being reached in September, Ukrainian and rebel troops engage on a regular basis, with some of the heaviest fighting focused on Donetsk airport.

One government paratrooper was killed Friday by a sniper at the airport, military authorities said in a statement. Polevoy said two other Ukrainian troops were killed on the same day, but gave no details.

The statement added that Ukrainian positions came under artillery fire in several towns and villages east of Donetsk, including Debaltseve, which has begun to be increasingly encircled by rebel forces.

Earlier this week, Ukraine's president Petro Poroshenko said that additional troops were being deployed to the east to defend cities still under government control against possible incursions. That followed rebel statements of intent to expand the amount of territory under their control.

The truce signed in the Belarusian capital, Minsk, by Russia, Ukraine and the separatists stipulates the pullback of heavy weaponry.

In Beijing, U.S. Secretary of State John Kerry and Russian Foreign Minister Sergey Lavrov met Saturday on the sidelines of the Asia-Pacific Economic Cooperation conference for what was expected to be a discussion about the unrest in eastern Ukraine.

Asked if Russia still respects the legitimacy of the cease-fire agreement, Lavrov said it is for the "rebels and the government" of Ukraine to finalize a disengagement line — a process that he said is continuing.

Tensions between Ukraine and Russia rose further after the rebels held an election last Sunday that Ukraine and the West denounced as a violation of the truce. Russia, however, quickly lent its support to the vote.

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Peter Leonard in Kiev, Ukraine, and Lara Jakes in Beijing contributed to this report.